I had a problem with the scanner on my SCX-4705. Before with Ubuntu 16.04 there was no problem but I just update to 18.04 and after it was hs..
After many hours I find a driver from ubuntu with synaptic. In french it's writen "pilote pour imprimantes laser samsung et xerox SPL2 et SPLc." : printer-driver-splix
I change many parameters but I beleive that it's the uninstall of this driver above that is the solution.
If it can help you ...
Thanks for sharing. It is not obvious to me why splix would be interfering, but that driver is now quite a bit older than the official drivers so some strange conflict is possible. I suspect it is more likely a change in CUPS that causes any conflict when both drivers are installed, but reverting CUPS is not practical. And there is certainly no need for splix if you are using the Samsung/HP driver.
Removing printer-driver-splix is safe, and hopefully it will help more people resolve the scanner issues.